Exopolysaccharide of Lachnum YM130 (LEP) was purified by diethylaminoethyl cellulose 52 and Sepharose CL-6B column chromatography. LEP-2a was identified to be a homogeneous component with an average molecular weight of 1.31 x 10(6) Da, which was consisted of mannose and galactose in a molar ratio of 3.8:1.0. The structure of LEP-2a was characterized by methylation analysis, FT-IR analysis, and NMR analysis. Results indicated that LEP-2a was a galactomannan with a backbone, composed of 1,2-linked-alpha-D-Manp, 1,2,6-linked-alpha-D-Manp, 1,3,4-linked-alpha-D-Manp, and 1,3-linked-beta-D-Galp, which was substituted at O-2, O-3, O-4, and O-6 by branches. In vitro antitumor activity assay proved that LEP-2a could significantly enhance the inhibitory effectiveness of 5-FU on Hela cells at the concentrations of 100, 200, 300, and 400 mu g/mL. The above results suggested that LEP-2a could be seen as a potential source for developing novel antineoplastic agents.